Many integrated circuit designs use a variety of cells that perform specific functions. Integrated circuits can include logic, memory, controller, and other functional blocks. Semiconductor integrated circuits are fabricated in a semiconductor process, often using a CMOS process. Transistors are formed in a semiconductor substrate and usually involve a sequence of fabrication steps that result in a gate with adjacent source and drain, with the source and drain being formed in a channel. A key setting for a transistor is the threshold voltage. A known method of setting threshold voltage is to place dopants in the channel area of opposite polarity from the dopants of the source and drain. Variations on channel doping include broadly imparting dopant material to fill up the volume of the channel. Other techniques include using pocket implants, also called halo doping, whereby ion implantation of channel dopants is limited to a small space that just adjoins each edge of the source and drain so that the remainder of the channel volume can remain substantially undoped so as to set the threshold voltage more precisely. As devices shrink, however, precision placement of dopants is increasingly difficult. Halo doping processes are imperfect and result in stray dopant material in unwanted locations in the channel area, making precision setting of threshold voltage very difficult. Threshold voltage variation has become an increasing problem and a limiter in rendering circuit designs that could otherwise take advantage of transistor size scaling. As a result of imprecise threshold voltage setting, while the scaling of transistor dimension has continued over time, the associated desired scaling down of supply voltage has not. The lack of scaling of power has hindered the ability for designers to create improved, reduced-power integrated circuits.
Low threshold voltage devices are generally used for high speed circuits, though low threshold voltage devices tend to have higher subthreshold leakage power. Designers, therefore, tend to design for higher threshold voltage. A common device design for higher threshold voltage is to impart dopants in the transistor channel that are of opposite polarity than the dopants in the source and drain. An advantage of providing dopants in the transistor channel is the relative improvement, in controlling short channel effects. As critical dimensions shrink, however, with greater relative number of dopants per unit volume in the channel, there may be more opportunity for junctions to form between the heavily doped source/drain regions and the channel region, creating a pathway for junction leakage.